The present invention relates to a liquid crystal display device and a fabrication method for the same, and more particularly, to a liquid crystal display device suitably used for portable information terminals (for example, PDAs), mobile phones, car-mounted liquid crystal displays, digital cameras, PCs, amusement equipment, TVs and the like.
The information infrastructure is advancing day to day, and equipment such as mobile phones, PDAs, digital cameras, video cameras and car navigators has penetrated deeply into people's lives. Liquid crystal display (LCD) devices have been adopted in most of such equipment. With increase of the information amount handled with the main bodies of the equipment, LCD devices are requested to display a larger amount of information, and are demanded by the market for higher contrast, a wider viewing angle, higher brightness, multiple colors and higher definition.
A vertical alignment mode using a vertically aligned liquid crystal layer has increasingly received attention as a display mode enabling high contrast and a wide viewing angle. The vertically aligned liquid crystal layer is generally obtained using a vertical alignment film and a liquid crystal material having negative dielectric anisotropy.
For example, Japanese Laid-Open Patent Publication No. 6-301036 (Literature 1) discloses an LCD device in which an inclined electric field is generated around an opening formed in a counter electrode that faces a pixel electrode via a liquid crystal layer, so that liquid crystal molecules surrounding liquid crystal molecules existing in the opening, which are in the vertically aligned state, are aligned in inclined directions around the opening as the center, to thereby improve the visual angle characteristics.
However, in the device described in Literature 1, it is difficult to generate an inclined electric field over the entire region of each pixel. Therefore, each pixel has a region in which liquid crystal molecules delay in response to a voltage, and this causes a problem of occurrence of an afterimage phenomenon.
Japanese Laid-Open Patent Publication No. 2002-55347 (Literature 2) discloses a technology in which slit electrodes (an opening pattern) are provided in both pixel electrodes and a counter common electrode and, in at least either the pixel electrodes or the common electrode, steps are provided in regions having the slit electrodes to distribute an inclined electric field in four directions uniformly using the opening pattern, to thereby achieve a wide viewing angle.
Japanese Laid-Open Patent Publication No. 2003-167253 (Literature 3) discloses a technology in which a plurality of projections are provided regularly in each pixel to stabilize the aligned state of liquid crystal domains having radially inclined alignment formed around the projections. This literature also discloses using an inclined electric field generated at openings formed in an electrode, together with the alignment regulating force of the projections, to regulate the alignment of liquid crystal molecules, and thus improve the display characteristics.
In recent years, a type of LCD device providing high-quality display both outdoors and indoors has been proposed (see Japanese Patent Gazette No. 2955277 (Literature 4) and U.S. Pat. No. 6,195,140 (Literature 5), for example). In this type of LCD device, called a transflective LCD device, each pixel has a reflection region in which display is done in the reflection mode and a transmission region in which display is done in the transmission mode.
The currently available transflective LCD devices adopt an ECB mode, a TN mode and the like. Literature 3 described above discloses adoption of the vertical alignment mode for, not only a transmissive LCD device, but also a transflective LCD device. Japanese Laid-Open Patent Publication No. 2002-350853 (Literature 6) discloses a technology in which in a transflective LCD device having a vertically aligned liquid crystal layer, the alignment (multi-axis alignment) of liquid crystal molecules is controlled with depressions formed on an insulating layer. The insulating layer is provided to make the thickness of the liquid crystal layer in a transmission region twice as large as that in a reflection region. According to this literature, the depressions are in the shape of a regular octagon, for example, and projections or slits (electrode openings) are formed at positions opposed to the depressions via the liquid crystal layer (see FIGS. 3 and 16 of Literature 6, for example).
Japanese Laid-Open Patent Publication No. 2001-125144 (Literature 7) discloses a technology of adopting dot inversion drive for a vertically aligned liquid crystal display device to thereby minimize roughness of display and achieve a high contrast ratio and wide viewing angle characteristics. In Literature 7, the dot inversion drive is executed at a predetermined pixel pitch, and distortion in an electric field generated in the boundary portions of adjacent pixel electrodes is used to define the aligned directions of liquid crystal molecules. This literature describes that the pixel electrode pitch is preferably not smaller than the thickness of the liquid crystal layer and also not larger than 20 μm, to make full use of the action of the electric field. The literature also describes that an opening is provided in the center of each pixel electrode to ensure fixation of the center of the radial alignment of liquid crystal molecules to the opening to thereby further stabilize the alignment.
The technology disclosed in Literature 3 has the following problems. Projections are provided in each pixel to form a plurality of liquid crystal domains in the pixel (that is, divide the pixel into domains), to thereby strengthen the alignment regulating force on liquid crystal molecules. According to examinations conducted by the inventors of the present invention, however, to obtain sufficient alignment regulating force, it is necessary to form an alignment control structure made of projections regularly placed inside each pixel, and this complicates the fabrication process. The contrast ratio may decrease due to light leakage occurring in the peripheries of the projections in the pixel. If a light-shading portion is provided to prevent decrease in contrast ratio, the effective aperture ratio may possibly decrease. Moreover, if the alignment regulating structure is provided on the counter substrate, the substrate alignment margin must be taken into consideration. Therefore, roughness of display due to deviation of the center axes of axisymmetrically aligned domains, and the decrease of the effective aperture ratio and/or the decrease of the contrast ratio will become further conspicuous.
In the technology disclosed in Literature 6, it is necessary to provide projections or electrode openings at positions opposed to the depressions formed for control of the multi-axis alignment. This technology therefore has the same problems as those described above.
In Literature 7, in which the directions in which liquid crystal molecules tilt are defined by use of a lateral electric field (an inclined electric field component in the substrate plane) generated between adjacent pixel electrodes by adopting the dot inversion drive, the stability of the aligned state is dependent on the shape of the electrodes. Also, sufficient alignment regulating force will not be exerted in a grayscale display state having a low electric field. Moreover, as is self-explanatory from the aligned directions of liquid crystal molecules existing in the four corners of each pixel electrode shown in FIG. 9 of Literature 7, the lateral electric field has directions different from each other (roughly orthogonal to each other) in each corner portion of the pixel electrode. This causes a problem of easily generating disclination (alignment defect) in the corner portions of each pixel.
In view of the above, an object of the present invention is providing a liquid crystal display device having at least one axisymmetrically aligned domain in each pixel, which can provide good axisymmetrically aligned domains even in a grayscale display state, is excellent in display quality and has wide viewing angle characteristics.